1. Field of the Invention
The invention relates generally to Global Positioning System (GPS) receivers and more particularly to a method and an apparatus for computing a precise location using differential carrier phases of a GPS satellite signal and a low earth orbiting (LEO) satellite signal.
2. Description of the Prior Art
Global Positioning System (GPS) receiver systems commonly use differential carrier phase measurements for finding a precise location vector for a geographical location of a GPS user receiver with respect to a geographical location of a GPS reference receiver. The GPS reference receiver and the GPS user receiver each determine a phase of the carrier of a GPS satellite signal received from two or more GPS satellites in common view of both GPS receivers. The location vector is calculated by taking the difference between the carrier phase determined at the GPS user receiver and carrier phase determined at the GPS reference receiver for the GPS satellite signal for two or more satellite geometries.
The differential carrier phase measurement must solve three appreciable problems: 1) the carrier phases are ambiguous until the number of wavelengths of the GPS satellite signal between the GPS satellite and each of the GPS receivers is resolved, 2) the ionospheric delay of the GPS satellite signal from each GPS satellite to each GPS receiver must be known, and 3) the reference carrier phase measurements and reference pseudoranges must be communicated from the GPS reference receiver to the GPS user receiver.
The carrier phase of the GPS satellite signal from each GPS satellite to each GPS receiver is ambiguous unless the number of wavelengths of the GPS satellite signal between the GPS satellite and the GPS receiver is known or its effect is eliminated. This problem, known as cycle resolution, integer determination, or resolution of the integer, is well-known and solutions are described in several U.S. Patents, such as U.S. Pat. No. 4,170,776 by MacDoran, U.S. Pat. No. 4,667,203 by Counselman, U.S. Pat. No. 4,963,889 by Hatch, U.S. Pat. No. 5,296,861 by Knight, and U.S. Pat. No. 5,519,620 by Talbot et al. Existing GPS receiver systems resolve the number of wavelengths by observing the GPS satellite signal for the same GPS satellites at two or more different satellite geometries. Traditionally, the observation requires thirty or more minutes in order to allow the GPS satellites to achieve a required geometric variation. Several techniques exist or have been proposed for reducing the observation time by using more intelligent satellite selection, precise pseudorange information, and/or carrier phase measurements of the GPS satellite signal at both L1 and L2 carriers. However, all of these techniques are limited by the slow rate at which the GPS satellites sweep across the sky.
The GPS satellite signal received by each GPS receiver must have approximately the same delay in the ionosphere or the difference in the delays must be accurately modeled in order to compute an accurate location vector. One solution to this problem is to locate the GPS user receiver and the GPS reference receiver close together, typically within a few kilometers, so that the ionospheric delay can be assumed to be the same to each GPS receiver. However, this solution limits the length of the location vector that can be determined. Another solution is to use a world-wide ionospheric model that is provided in the GPS satellite signal. However, the world-wide ionospheric model that is provided typically only accounts for half of the effect of ionospheric delay. Another solution is to receive the GPS satellite signal at both an L1 frequency and an L2 frequency as the GPS satellites traverse across the sky. Then, because the ionospheric delay of a signal is proportional to the inverse square of frequency of the signal, an ionospheric model can be estimated that is more accurate locally than the world-wide model. Unfortunately, the GPS receivers capable of receiving both the L1 and L2 frequencies are relatively expensive. Further, because of the long time for the GPS satellite to traverse the sky, the estimate loses accuracy during times of day when the ionosphere is changing rapidly.
Existing GPS receiver systems operate in real-time by transmitting carrier phase and pseudorange measurements from the GPS reference receiver to the GPS user receiver. Typically, the transmission is accomplished with terrestrial radios operating in the VHF or UHF frequency range. Unfortunately, these radios are subject to interference, multipath, shadowing, line-of-sight limits, and restrictive licensing requirements. Although satellite communication has been used for broadcasting the pseudorange measurements, it has not been seriously considered for broadcasting carrier phase measurements because the carrier phase measurements age too much during the time of travel up to the satellite and back down to Earth and in the latency in the satellite itself for determining the location vector with a required accuracy.
There is a need for a GPS receiver system and a method for improving the speed of resolving the integer number of wavelengths, for a rapid determination of a local ionospheric delay model, and for robust communication of carrier phase measurements between a GPS reference receiver and a GPS user receiver over a wide geographical area.